Yes, a large barn-door windscreen will reduce drag and increase fuel economy. When I first got my 2002 BMW R1150R I put a 22" Parabellum Sport / Touring screen on the bike. When I took it off and replaced it with a small national cycle screen the average fuel economy dropped a couple MPG. (~ 42 to 40 mpg)
I got rid of the Parabellum because if I got a gust of crosswind just right it would shake the bars and I don't like the feeling of a windshield steering the bike at 70 - 80 mph. It was also hot in the South in the summer (but nice when riding to work in the winter with temps in the 30's)
The clear National Cycle shield was a good compromise (This set-up was great for commuting and two-up touring. I would dare say better than the R1200RT I have today although I really like the electronic adjusting windscreen on the RT)
Parabellum
National Cycle
National Cycle (Clear)
